In recent times, probes for coordinate measuring apparatus have become known which apply only a very slight measuring force of less than 1 mN to the workpiece to be measured. These probes operate with a so-called solid-state oscillator having an amplitude of vibration which changes when approaching the object to be measured. Such a probe is disclosed in German utility model 9,213,059.
This known probe has a thin glass rod as a probe element which is attached with adhesive to a prong of a tuning fork crystal in such a manner that the longitudinal axis of the glass rod is aligned parallel to the direction of vibration of the tuning fork crystal.
When a probe of this kind is used in a coordinate measuring apparatus and when the longitudinal axis of the rod or probe element is aligned parallel to one of the measuring directions of the coordinate measuring apparatus as shown in FIG. 1, it can then be determined that the attainable contact uncertainties in the coordinate direction z (that is, in the longitudinal direction of the rod 3a and parallel to the direction S of vibration of the tuning fork crystal) are significantly less than the contact uncertainties which are measured in the coordinate directions x and y perpendicular to the longitudinal axis of the rod 3a. The measuring uncertainties lie apart from each other by a factor of 5. While, for example, a contact uncertainty of only 1.0 .mu.m was determined for the measuring axis z, this uncertainty amounts to about 5 .mu.m for the contact directions x and y.
An asymmetry of this kind in the contact uncertainty is, however, unwanted because many sides must be contacted when making measurements of geometric elements such as bores, et cetera. The measurement uncertainty should correspond for all contact points to the same value specified for the measuring apparatus.
A further problem when working with the above-mentioned probe is obtaining signals which announce clearly and reliably the contact of time probe element with the workpiece to be measured to the control of the coordinate measuring apparatus. If this time point is not clearly and reproducibly determined, then this likewise increases the contact uncertainty of the probe. The circuit described in German utility model 9,213,059 does supply usable results. However, the circuit is rather sensitive to external disturbances. For example, the transfer of measured values is influenced by turbulences in the air. When such disturbances occur shortly ahead of contact on the workpiece surface, then these disturbances are not clearly recognized as disturbances and a false coordinate measuring value is supplied, that is, these disturbances contribute to an increased contact uncertainty.